The present invention is directed to a closure for use in closing between spaced arms of a medical implant and securing a rod to the implant. In particular, the closure includes a non-circular multi-surfaced or multi-lobular internal bore for improved engagement by a complementary shaped tool for purposes of removal and an interlocking helical guide and advancement structure that prevents splaying of upper ends of walls of the implant within which the closure is placed away from an axis of rotation of the closure.
Medical implants present a number of problems to both surgeons installing implants and to engineers designing them. It is always desirable to have an implant that is strong and unlikely to fail or break during usage. It is also desirable for the implant to be as small and lightweight as possible so that it is less intrusive on the patient. These are normally conflicting goals, and often difficult to resolve.
One particular type of implant presents special problems. In particular, spinal bone screws, hooks, etc. are used in many types of back surgery for repair of injury, disease or congenital defect. For example, spinal bone screws of this type are designed to have one end that inserts threadably into a vertebra and a head at an opposite end. The head is designed to receive a rod or rod-like member in a channel in the head in which the rod is both captured and locked to prevent relative movement between the various elements subsequent to installation. The channel in the head is open ended and the rod is simply laid in the open channel. The channel is then closed with a closure member. The open headed bone screws and related devices are much easier to use and in some situations must be used instead of closed headed devices.
While open headed devices are often necessary and often preferred for usage, there is a significant problem associated with them. In particular, the open headed devices conventionally have two upstanding arms that are on opposite sides of a channel that receives the rod member. The top of the channel is closed by a closure after the rod member is placed in the channel. The closure can be of a slide in type, but such are not easy to use. Threaded nuts are sometimes used that go around the outside of the arms. Such nuts prevent splaying of the arms, but nuts substantially increase the size and profile of the implant which is not desirable. Many open headed implants are closed by plugs, bodies or closures that screw into threads between the arms, because such have a low profile. However, threaded plugs have encountered problems also in that they produce radially outward directed forces that lead to splaying or spreading of the tops of the arms or at least do not prevent splaying caused by outside forces that in turn loosen the implant. In particular, in order to lock the rod member in place, a significant force must be exerted on the relatively small plug. The tightening forces are required to provide enough torque to insure that the rod member is clamped or locked in place relative to the bone screw, so that the rod does not move axially or rotationally therein. Torques on the order of 100 inch-pounds are typical.
Because open headed implants such as bone screws, hooks and the like are relatively small, the arms that extend upwardly at the head can rotate relative to the base that holds the arms so that the tops of the arms are rotated or bent outward relatively easily by radially outward directed forces due to the application of substantial forces required to secure the rod member. Historically, early closures were simple plugs that were threaded with V-shaped threads and screwed into mating threads on the inside of each of the arms. But, as noted above, conventional V-shaped threaded plugs tend to splay or push the arms radially outward upon the application of a significant amount of torque, which ends up bending the arms sufficiently to allow the threads to loosen or disengage and the closure to fail. To counter outward directed application of forces, various engineering techniques were applied to resist the spreading forces. For example, the arms were significantly strengthened by substantially increasing the width of the arms. This had the unfortunate effect of substantially increasing the weight and the profile of the implant, which was undesirable.
The tendency of the open headed bone screw to splay is a result of the geometry or contour of the threads typically employed in such devices. In the past, most bone screw head receptacles and screw plugs have employed V-shaped threads. V-threads have leading and trailing sides oriented at angles to the screw axis. Thus, torque on the plug is translated to the bone screw head at least partially in an axial outward direction, tending to push or splay the arms of the bone screw head radially outward. This in turn spreads the internally threaded receptacle away from the thread axis so as to loosen the plug in the receptacle. The threads also have smooth or linear surfaces in a radial direction that allow slippage along the surfaces since they at best fit interferingly with respect to each other and have in the past not interlocked together. Thus, forces other than insertion forces can act to easily splay the arms since the surfaces slide rather than interlock.
The radial expansion problem of V-threads due to the radial outward component of forces applied to a V-thread has been recognized in various types of threaded joints. To overcome this problem, so-called “buttress” threadforms were developed. In a buttress thread, the trailing or thrust surface is oriented perpendicular to the thread axis, while the leading or clearance surface remains angled. This theoretically results in no radially inward or outward directed forces of a threaded receptacle in reaction to application of torque on the threaded plug. However, the linear surfaces still allow sideways slippage, if other forces are applied to the arms.
Development of threadforms proceeded from buttress threadforms which in theory have a neutral radial force effect on the screw receptacle, to reverse angled threadforms which theoretically positively draw the threads of the receptacle radially inward toward the thread axis when the plug is torqued. In a reverse angle threadform, the trailing side of the external thread is angled toward the thread axis instead of away from the thread axis, as in conventional V-threads. While buttress and reverse threadforms reduce the tendency to splay, the surfaces are not interlocking and the arms can still be bent outward by forces acting on the implant. The threads can be distorted or bent by forces exerted during installation. Therefore, while these types of threadforms are designed to not exert radial forces during installation, at most such threadforms provide an interference or frictional fit and do not positively lock the arms in place relative to the closure plug.
Furthermore, it is noted that plugs of this type that use threadforms are often cross threaded. That is, as the surgeon tries to start the threaded plug into the threaded receiver, the thread on the plug is inadvertently started in the wrong turn or pass of the thread on one arm. This problem especially occurs because the parts are very small and hard to handle. When cross threading occurs, the plug will often screw part way in the receiver and then “lock up” so that the surgeon is led to believe that the plug is tight and properly set. However, the rod is not secure relative to the bone screw or other implant and the implant fails to function properly. Therefore, it is also desirable to have a closure that resists cross threading in the receiver.
As stated above, it is desirable for medical implants to have strong and secure elements which are also very lightweight and low profile so that the overall implant impacts as little as possible upon the patient. However, strong and secure are somewhat divergent goals from the goals of lightweight and low profile. Thus, size, weight, and profile must all be taken into consideration and minimized, as much as possible, consistent with effective functioning.
In order to provide sufficient strength and friction to resist movement of the various elements once the closure plug is seated, it is necessary to apply a fairly substantial amount of torque to the closure. While some closure plugs are torqued without a head, many of the closure plugs currently in use in medical implants have a driving or installation head that breaks away from the remainder of the fastener at a preselected torque in order to assure that the closure is sufficiently torqued to provide the necessary strength and locking friction. The head is also broken away in order to assure that the closure is not over-torqued. Further, the head is typically broken away in order to provide the low profile and light weight that is desired in such closure plugs.
Because the driving head is typically broken away and because it is sometimes necessary to remove the closure after implantation and setting thereof, some mechanism must be provided in order to securely engage and remove the closure. Various structures have been provided for this purpose in prior art devices. The prior art structures have had varying degrees of success, but have typically been most effective in fasteners having a diameter that is comparatively large, such as 9 to 12 millimeters, because such larger fasteners provide greater surface and volume for engagement by removal structure of one kind or another. However, it is desirable to provide an implant closure plug with a removal mechanism which works effectively with implant elements of even smaller size.